我會放棄提及的范圍,因為它真的不是很相關.

with EXPR as VAR:

BLOCK

翻譯成

mgr = (EXPR)

exit = type(mgr).__exit__ # Not calling it yet

value = type(mgr).__enter__(mgr)

exc = True

try:

try:

VAR = value # Only if "as VAR" is present

BLOCK

except:

# The exceptional case is handled here

exc = False

if not exit(mgr, *sys.exc_info()):

raise

# The exception is swallowed if exit() returns true

finally:

# The normal and non-local-goto cases are handled here

if exc:

exit(mgr, None, None, None)

如你所見,鍵入(mgr).__ enter__被調用,如你所料,但不是在嘗試.

類型(mgr).__ exit__在退出時調用.唯一的區別是當有異常時,如果沒有退出(mgr,* sys.exc_info())路徑.這使得能夠內省和沉默錯誤,而不像finally子句可以做什么.

contextmanager并不會使這一點復雜化.只是：

def contextmanager(func):

@wraps(func)

def helper(*args, **kwds):

return _GeneratorContextManager(func, *args, **kwds)

return helper

然后看看有關的課程：

class _GeneratorContextManager(ContextDecorator):

def __init__(self, func, *args, **kwds):

self.gen = func(*args, **kwds)

def __enter__(self):

try:

return next(self.gen)

except StopIteration:

raise RuntimeError("generator didn't yield") from None

def __exit__(self, type, value, traceback):

if type is None:

try:

next(self.gen)

except StopIteration:

return

else:

raise RuntimeError("generator didn't stop")

else:

if value is None:

value = type()

try:

self.gen.throw(type, value, traceback)

raise RuntimeError("generator didn't stop after throw()")

except StopIteration as exc:

return exc is not value

except:

if sys.exc_info()[1] is not value:

raise

不重要的代碼已經消失.

首先要注意的是,如果有多個收益率,這個代碼將會出錯.

這并不明顯地影響控制流程.

考慮__enter__.

try:

return next(self.gen)

except StopIteration:

raise RuntimeError("generator didn't yield") from None

如果上下文管理器寫得很好,那永遠不會從預期的中斷.

一個區別是,如果生成器拋出StopIteration,將產生一個不同的錯誤(RuntimeError).這意味著,如果您正在運行完全任意的代碼,則該行為與正常行為不完全相同.

考慮一個非錯誤的__exit__：

if type is None:

try:

next(self.gen)

except StopIteration:

return

else:

raise RuntimeError("generator didn't stop")

唯一的區別就像以前一樣.如果你的代碼拋出StopIteration,它會影響生成器,因此contextmanager裝飾器會誤解它.

這意味著：

from contextlib import contextmanager

@contextmanager

def with_cleanup(func):

try:

yield

finally:

func()

def good_cleanup():

print("cleaning")

with with_cleanup(good_cleanup):

print("doing")

1/0

#>>> doing

#>>> cleaning

#>>> Traceback (most recent call last):

#>>> File "", line 15, in

#>>> ZeroDivisionError: division by zero

def bad_cleanup():

print("cleaning")

raise StopIteration

with with_cleanup(bad_cleanup):

print("doing")

1/0

#>>> doing

#>>> cleaning

哪個不太重要,但可以.

最后：

else:

if value is None:

value = type()

try:

self.gen.throw(type, value, traceback)

raise RuntimeError("generator didn't stop after throw()")

except StopIteration as exc:

return exc is not value

except:

if sys.exc_info()[1] is not value:

raise

這提出了與StopIteration相同的問題,但有趣的是注意到最后一部分.

if sys.exc_info()[1] is not value:

raise

這意味著如果未處理異常,則追溯將不會更改.如果它被處理但存在新的追溯,那將被提升.

這完全符合規格.

TL; DR

與實際上比實際上稍微強大一點…終于可以內省和沉默錯誤.>請注意StopIteration,否則您可以使用@contextmanager創建上下文管理器.

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